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Published July 24, 2009, 12:00 AM

Fungus, not borers, may be plant’s problem

Q: I have a hollyhock that I think has corn borers. It has many yellow and orange spots on the leaves, and there are noticeable bumps on the undersides of the leaves.

By: Don Kinzler, INFORUM

Q: I have a hollyhock that I think has corn borers. It has many yellow and orange spots on the leaves, and there are noticeable bumps on the undersides of the leaves. I read from your posting that you recommend orthene. I assume this is a systemic, so I’m wondering if this might be toxic to bees or butterflies that might get on the flowers. The plant hasn’t bloomed, but the flower buds seem healthy along with the rest of the plant, but unsightly. Don’t want them to spread to others nearby though. Thanks so much for any help. (e-mail reference)

A: Any systemic material will get into the flower parts, as well as the leaves and stems. If the concentration is strong enough to harm the pollinators, I can’t say. From your description, it doesn’t sound like boring insects, but rather a fungal disease, such as rust. Get a fungicide, such as daconil, applied before the flowers open. That way you won’t harm the pollinators.


Q: Can you tell me what I should do for my spider plant that has many bending and creased leaves? The leaves are about 20 inches long, but some of them bend in the middle. Should I be feeding it? It is otherwise healthy, but has a few brown spots on the leaf tips. My friend recommended putting it outside for some sun, but I would prefer to have it indoors. I did move it outside to a spot that gets very indirect sunlight. I also fed it. Will this help strengthen and straighten the leaves? Can I move it back inside and give it indirect sunlight? Thank you very much for your help. (e-mail reference)

A: Do nothing. This plant’s normal home is not in a pot. It grows in a tropical setting as a ground cover. Fertilizing it once a month during the summer growing period is highly recommended using a water-soluble fertilizer. Yes to all of your other questions. There isn’t a houseplant in America that doesn’t perk up when summered outdoors.


Q: My husband and I just bought a house, so I’m wondering if I can plant my three potted spider plants outside in the yard somewhere. If I can, where would the best place to plant them?

(e-mail reference)

A: I wish people would tell me where they live when they correspond with me. Assuming you live somewhere in North America that is not tundra, you can go ahead and plant the spider plants outdoors for the summer. Leave the plants in the pots, preferably in dappled (under a deciduous tree) shade. Water the plants two to three times a week, depending on rainfall frequency. Be sure to get them indoors again before the first frost.


Q: We have a prairie fire crab in our front yard. It was planted by a nursery about four years ago. This year, I see that a few of the branches aren’t bearing leaves or blossoms. It looks like the leaves started to come out, but then stopped. The rest of the tree seems fine and the bare branches aren’t brittle. Could the spring freeze we experienced here in the Twin Cities be to blame? Will the branches come back on their own? (e-mail reference)

A: It is not very likely. This selective death of the branches is caused by borer activity or canker growth that has girdled the affected branches. To make a positive determination, examine the branches closely. Look for small holes in the bark. If you find some, cut into some of the holes to see if larvae have created galleries under the bark that has girdled the branch. If you don’t find any holes, look for sunken areas. The sunken areas would indicate the presence of a fungal canker. In either case, remove the affected branches and take protective action on the rest of the tree with the appropriate insecticide or fungicide.


Q: I have two rhubarb patches. One is strong and healthy and of the green stalk variety. My other patch is a red stalk variety. Last summer, this second patch died after producing an abundance of rhubarb. Just before its decline, the leaves began to be covered in massive red spots. I wasn’t sure if the plant was going to grow this season. It has, but at an extremely slow rate. The stalks are red and finger-thin, plus the red spots are back on the leaves. What is this condition called and how should I take care of it? I think the plants are at least a decade old, if not older. (e-mail reference)

A: Due to the cool, wet spring, I have seen a number of diseased rhubarb leaves this year. Most of the samples are infected with ascochyta leaf spot or anthracnose stalk rot. The first indications of ascochyta leaf spot are the numerous small, yellowish-green areas on the upper surface of the leaves. Within a week of initial symptoms, the leaf tissue turns brown and dies, leaving angular spots. These spots have white centers surrounded by a red zone and then a grayish-green zone. Often the dead tissue will drop out of the leaves. This gives the leaves a shot-hole appearance that may be confused with insect feeding. A second disease problem that has been common this year is anthracnose stalk rot. First indications of this disease are wilted leaves and large, water-soaked lesions on the stems. The lesions quickly enlarge and turn black. The stems may have a twisted appearance and the whole stem may collapse.

Both of these diseases can be controlled using good sanitation practices. Remove and dispose of infected tissue during the summer and after the first frost. In the case of ascochyta leaf spot, stems with infected leaves still may be harvested and should be taken first whenever possible. Since both diseases overwinter in infected plant tissue, good sanitation practices should control most of the disease problems. In addition to good sanitation practices, the plants should be heavily fertilized as soon as new growth appears next spring. Another mild application should be made as soon as the stalk harvest is completed. These two diseases are more prevalent in plants that are stressed, so any measures to improve their growing conditions should decrease injury problems caused by disease. If the plants are as old as you think they are, they should be divided and replanted. This can be done early in the spring just as the frost is leaving the soil.


Q: I have two weigela shrubs that are two years old and not blooming. They did grow a few flowers here and there. What can I do to help them bloom or what am I doing wrong? I am ready to give up. (e-mail reference)

A: Weigela shrubs require a particular pruning to keep the blooms coming on strong. Giving a shrub an annual late-spring pruning to remove old or damaged wood will encourage new growth to come forth and be the source for vigorous flowering. Weigela is one of the nonericaceous plants that need well-drained soil that is rich in organic matter. If the soil is poorly drained and remains soggy, it will be a poor performer. Like most flowering shrubs, weigela needs to have ample sunlight, which is at least six hours of direct sunlight a day.


Q: I have grown cucumbers for many years with good luck. This year, I planted some pickling cucumbers that are growing wonderfully and have many flowers. The flowers produce a small cucumber about an inch long, but then turn yellow and fall off. This is not happening to my straight eights. Is this something that can be weather related? We have been having another wet year, but now the heat has turned on. (Wichita, Kan.)

A: It could be weather related or that they simply were not fertilized completely, which caused the fruit to abort. Keep in mind that the straight eights cucumbers are parthenocarpic, which means they are a seedless variety that don’t require pollination activity. This makes them more immune to the vagaries of the weather.


Q: I am very new to growing tulips. I bought my bloomed tulips around Easter. I transferred them to a bigger pot filled mostly with clay (that’s all I had at the time). After they wilted and turned yellow, I cut off the dying stems and leaves. About all I had left was dry clay in a pot. My dogs hit the pot and broke it. I quickly went and got fresh soil and repotted the bulbs, but they were soggy and falling apart. I saved a few inside parts of the bulb that were hard and replanted them. Is it too late? What should I have done? Will they bloom again? Thanks for your time. (Louisiana)

A: These were forced bulbs that were not intended to be reused in your part of the country. They are a one-time show and then dumped. The bulbs rotted because the soil was tight clay and you kept them moist too long. Throughout the southern U.S. where the winter temperatures don’t get low enough or the summer weather gets too hot too soon, bulbs can be grown only if they are pretreated with an adequate low temperature for a long enough time. This usually takes place in refrigerated units. Then the bulbs are planted in pots or landscape settings. Once the blooms fade, the bulbs are disposed of.


Q: We recently laid down new sod in our backyard. Some of the sod was yellowing when installed. One week later, about 70 percent of the sod is green and nice, but you can see the strips of sod that are yellow and haylike. (St. Cloud, Minn.)

A: This usually happens when the sod overheats. Keep in mind that this is essentially green manure and will heat up quickly just like a compost pile when new grass clippings are mixed in. Kept too long in a stack or roll, the heat can get to lethal temperatures. If it doesn’t start to green up in another couple of weeks, get back to the contractor who laid the sod and request replacement pieces. I’d suggest taking photos of what you are talking about and documenting this with the sod contractor. In the meantime, keep up with the normal watering regime without overdoing it. If the sod was not killed from elevated temperatures, it should recover with normal watering.


Gardening or houseplant questions can be directed to: Hortiscope, Box 6050, NDSU Dept. 7670, Fargo, ND 58108-6050 or ronald.smith@ndsu.edu

Note to e-mail correspondents: please identify your location (city and state) for most accurate recommendations

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